Thursday, November 5, 2009

More frequencies in FFT

Hi Transmogrifox, thank you very much for your help. We have this blog to interact and to help ourselves.

The Transmogrifox's code is given following:


//Frequency components of a signal
//----------------------------------
// build a noides signal sampled at 1000hz containing to pure frequencies
// at 50 and 70 Hz
sample_rate=1000;
t = 0:1/sample_rate:0.6;
N=size(t,'*'); //number of samples
s=sin(2*%pi*50*t)+ 0.3*sin(2*%pi*70*t+%pi/4)+ 0.2*grand(1,N,'nor',0,1);

y=fft(s);
//the fft response is symetric we retain only the first N/2 points
f=sample_rate*(0:(N/2))/N; //associated frequency vector
n=size(f,'*')
yy = y*2/N;
clf()
plot2d(f,abs(yy(1:n)))


And the following picture is the result.



So, if anyone has any question, I think I and Transmogrifox are ready for help.

2 comments:

mc008 said...

Hi, it has been really useful, thanks. But there' s a line I don't understand:

yy = y*2/N;

Would you explain it please?

Sheep said...

Hi Mc008, how are you?

The line:
yy = y*2/N;

makes a normalization of 'y' and attributes the result to 'yy'.

If you use the value of 'y' directly, the FFT's amplitude would be N/2 times greater than the normal, but if you see the picture, the result showed has the picks exactly to 1 in frequency of 50 and 0,3 in frequency of 70.

I hope to have helped.